This invention relates to a method for reducing the soluble and insoluble nickel impurities in waste water. More specifically, it relates to a process for reducing the total nickel content of waste water, such as grey water or blow-down water from a partial oxidation process, to produce an environmentally acceptable discharge stream, or to prevent the build-up of nickel in the circulating water system and the formation of nickel carbonyl.
Nickel from soluble and insoluble nickel compounds may be present in dilute waste water from chemical processes such as from the partial oxidation process in the amount of about 1 to 15 milligrams per liter (mg/l). When such water is recycled in the process, the nickel concentration in the system may greatly increase. Environmental regulations may require that prior to discharge of such water into a sewer, the total concentration of nickel from water soluble and insoluble nickel compounds be reduced to less than 1 mg/l.
In the process for producing synthesis gas, reducing gas, or fuel gas by the partial oxidation of a liquid hydrocarbonaceous fuel, such as a liquid hydrocarbon fuel, oxygen-containing hydrocarbonaceous fuel or a slurry of a solid carbonaceous fuel with a water and/or liquid hydrocarbon carrier, it may be necessary to reduce the total nickel content in the grey water from the decanter, or blow-down water from the gas quench cooling and/or scrubbing operation, to an environmentally acceptable level prior to discharge from the system. Further, it is desirable to prevent the build-up of nickel in the quench cooling and/or scrubbing circulating water system in the partial oxidation process in order to avoid the formation of volatile nickel carbonyl. Nickel carbonyl may decompose and form undesirable nickel deposits on heat exchanger walls. For example, if traces of the H.sub.2 S remain in the gas, nickel sulfide may accumulate on the heat exchanger walls with deleterious effects on the metallurgy. Further, in low temperature catalytic synthesis, nickel carbonyl may decompose in mixtures of hydrogen and carbon monoxide. Nickel may then accumulate in homogenous and heterogeneous noble metal catalyst systems and adversely affect the selectivity and activity of the catalyst.
Coassigned U.S. Pat. No. 4,211,646, which is incorporated herein by reference, relates to a process for removing cyanide, sulfide, ammonia, soot and insoluble metals in the scrubbing water from the partial oxidation process. Ferrous sulfate and lime addition, followed by clarification, filtration, and steam stripping are employed. However, it was found that soluble nickel cannot be consistently reduced to very low limits by that process.
It was unexpectedly found that by the subject process, the concentration of nickel in waste water, such as in grey water from the decanter in the carbon recovery system of the partial oxidation process, or blow-down water from the gas quench cooling and/or scrubbing operation, may be consistently reduced to environmentally acceptable levels e.g. less than 1 mg/l, such as below about 0.5 mg/l. Further, these low levels could not be consistently achieved by merely adding dimethyl glyoxime to the grey water.